This invention relates to a control device for a direct-coupling hydraulic clutch provided in a hydraulic torque converter of an automotive vehicle, and more particularly to a control device of the type employing an electromagnetic valve as a pilot valve and adapted to selectively render the direct-coupling hydraulic clutch operative or inoperative by means of the electromagnetic valve.
An automotive vehicle equipped with a hydraulic torque converter can provide a smooth driving characteristic with its speed reduction ratio smoothly changeable, but is inferior to one equipped with a manual transmission in the fuel consumption of the engine due to slippage inherent in the hydraulic torque converter. To eliminate this disadvantage, it has been proposed to mechanically lock up the torque converter to reduce the slip loss to a minimum required value when the amplification of torque performed by the torque converter is almost not available.
Further, a method of controlling a direct-coupling hydraulic clutch in a torque converter has been proposed, e.g. by Japanese Patent Application No. 58-25064 assigned to the assignee of the present application, which is adapted to mechanically lock up the torque converter in a manner increasing the engaging force of the direct-coupling clutch by locking up the torque converter in proportion to an increase in the vehicle speed, instead of fully locking up same, while allowing slippage in the torque converter in a low vehicle speed region, to thereby absorb vibrations of the engine, and at the same time to maintain the torque amplifying function of the torque converter.
In a direct-coupling clutch of the type employing an electromagnetic valve for effecting or interrupting mechanical locking-up of the torque converter, if the electromagnetic valve is used as a pilot valve and at the same time part of the operating fluid for actuating the direct-coupling clutch is used as pilot pressure fluid, it will be necessary to drain part of the pilot operating fluid either during the engagement of the direct-coupling clutch to lock up the torque converter or during disengagement of the same clutch. If the pilot operating fluid is permitted to drain during actuation of the direct-coupling clutch, the hydraulic system concerned has to be designed so as to compensate for a drop in the line pressure, which can thus occur during running of the vehicle. Besides, it will be necessary to design the electromagnetic valve so as to fully withstand a great deal of heat generated by itself due to its very long energization period. As a result, the electromagnetic valve will inevitably be large in size. Further, a hydraulic pump with a large capacity is required in the hydraulic system to compensate for drainage of the operating fluid during running of the vehicle, which leads to increased consumption of the electric power, thus being impractical. On the other hand, if part of the pilot operating fluid is permitted to drain during interruption of the locking-up of the torque converter, the direct-coupling clutch can wrongly be actuated to lock up the torque converter when a disconnection occurs in the electromagnetic valve per se or its wiring system, causing engine stall upon stoppage of the vehicle.
Since engine stall due to disconnection of the electromagnetic valve should be avoided first of all, conventionally it has been generally employed to permit part of the operating fluid to drain during locking-up of the torque converter, by employing a hydraulic pump with a large capacity in the hydraulic system.